Much of the post-Sept. 11 anxiety about future acts of terrorism has focused on the threat of biological or nuclear disaster — but an attack on a U.S. chemical plant could do as much or more harm, killing or injuring as many as 2.4 million people. That figure, which comes from a report by the Army surgeon general obtained by the Washington Post, is twice as high as earlier government estimates. More than 100 chemical plants in the U.S have sufficient stockpiles of chemicals to cause damage on that scale. Environmental and public safety groups have called for increased security measures at chemical plants, but Chris VandenHeuvel, a spokesperson for the American Chemistry Council, called the industry “one of the safest manufacturing sectors in America.”